This invention relates to a method of recovering high-purity sodium bichromate, potassium bichromate and/or bichromic acid from a chromic acid solution containing impurities.
Conventional production and recovery methods of chromate comprised roasting chromium ores with NaOH, Na.sub.2 CO.sub.3 or Na.sub.2 SO.sub.4, leaching them with water to obtain a sodium chromate solution and crystallization of sodium bichromate by concentration after a purification process. Another conventional process comprises an addition of KCl to a sodium bichromate solution to obtain a potassium bichromate and an addition of H.sub.2 SO.sub.4 to a sodium bichromate to obtain an anhydrous chromic acid. These high-purity products can be obtained owing to the content of separable substances in the production of sodium chromate in the case of a chromate solution containing a comparatively small amount of impurity. However, in case of chromium plating or chromate treatment solution containing a large amount of heavy metallic ion or Na.sub.2 SiF.sub.6 and NH.sub.4 HF, etc., an economical disadvantage is brought on by the increase of purification cost.
An extraction method in which CrO.sub.4.sup.2- ions in the aqueous solution are extracted into an organic phase as their adducts with contact of TBP (tri-butyl phosphate) or TOPO (tri-octyl phosphine oxide) and a strip method in which H.sub.2 Cr.sub.2 O.sub.7 or H.sub.2 CrO.sub.4 extracted into the organic phase is stripped with NaOH or Na.sub.2 CO.sub.3 solution have been published, but an additional treatment is necessary due to the unfavourable problem of public pollution caused by 0.1-1.0 g/l of CrO.sub.4.sup.2- ion existing in the raffinate.
Moreover, CrO.sub.4.sup.2- ions have a tendency to be reduced by long residence in the organic phase and the stable operation can not be maintained owing to the formation of the third phase by Cr(OH).sub.3 yielded in a strip process with NaOH, etc. Furthermore, the solutions which are used in etching of ABS resin, etc. and contain high concentration of CrO.sub.4.sup.2- ion and H.sub.2 SO.sub.4 severely oxidize the above TBP or TOPO with direct contact and consequently an organic solvent, such as TBP or TOPO can not be used satisfactorily.
An extraction method in which CrO.sub.4.sup.2- ions in the aqueous solution are extracted with contact of an organic solvent containing high molecular weight amines has been published but this method has the following disadvantages:
(1) enhancement of CrO.sub.4.sup.2- ion extracted into the organic phase is impossible because the extreme increase of high molecular weight amine concentration is impossible in the organic phase,
(2) recycle to chromium-plating or chromate treatment is impossible owing to the form recovered as Na.sub.2 CrO.sub.4 by stripping with NaOH or Na.sub.2 CO.sub.3 solution.
It has been published also that extraction of CrO.sub.4.sup.2- ions in the chloric acid solution, for example, extraction and removal of Na.sub.2 CrO.sub.4 added to protect the anode in electrolysis of NaCl solution in a production process of chlorate was tried using a primary or secondary amine to improve product-purity and to prevent the secondary public pollution, but resulted in failure. The only successful extraction and removal process of CrO.sub.4.sup.2- ions in chloric acid solution using ion exchange resin is published in U.S. Pat. No. 3,980,751, but this process has also disadvantages, such as, repeated frequency of regeneration of the resin due to the small exchange capacity.
An adsorption-removal process of all cations from waste acids using cation exchange resin has been reported but the process is unfavourable due to the following disadvantages:
(1) repeated frequency of regeneration due to deficiency of exchange capacity
(2) a large amount of CrO.sub.4.sup.2- ions in the aqueous solution discharged by washing and regeneration processes
(3) significant degradation of resin
(4) problem of public pollution.
Morover, another adsorption-removal process of CrO.sub.4.sup.2- ions from waste solutions using anion exchange resin has been published but this process also has several disadvantages as follows:
(1) repeated frequency of regeneration owing to the lower exchange capacity and coadsorption of other anions coexisting with CrO.sub.4.sup.2- ions, such as SO.sub.4.sup.2- and Cl.sup.- ions
(2) impossible to continually obtain a constant concentration of CrO.sub.4.sup.2- ions in the regenerated solution,
(3) large amounts of reagents required for desorption, such as NaOH, etc.
(4) large amounts of other anions mingling with CrO.sub.4.sup.2- ions
(5) large amounts of water required for washing
(6) difficulty in formation of complete circuit of CrO.sub.4.sup.2- ions because they flow off in the washing solution.